For the purpose of reducing the weight of an automobile or ensuring the safety of passengers, cold-rolled steel sheets having such a high tensile strength as to permit achievement of a higher strength and reduction of the weight of various structural members, are widely used as materials for protective components of an automobile such as a bumper reinforcement and a door guard bar. As a cold-rolled steel sheet having such a high tensile strength, ultra-high-strength cold-rolled steel sheets having a tensile strength of over 100 kgf/mm.sup.2 are proposed as follows:
(1) an ultra-high-strength cold-rolled steel sheet, disclosed in Japanese Patent Provisional Publication No. 61-3,843 published on Jan. 9, 1986, which consists essentially of:
carbon (C): from 0.02 to 0.30 wt. %,
silicon (Si): from 0.01 to 2.5 wt. %,
manganese (Mn): from 0.5 to 2.5 wt. %,
and
the balance being iron (Fe) and incidental impurities
(hereinafter referred to as the "prior art 1").
(2) an ultra-high-strength cold-rolled steel sheet, disclosed in Japanese Patent Provisional Publication No. 61-217,529 published on Sep. 27, 1986, which consists essentially of:
carbon (C): from 0.12 to 0.70 wt. %,
silicon (Si): from 0.4 to 1.0 wt. %,
manganese (Mn): from 0.2 to 2.5 wt. %,
soluble aluminum (Sol.Al): from 0.01 to 0.07 wt. %,
nitrogen (total N): up to 0.02 wt. %,
and
the balance being iron (Fe) and incidental impurities
(hereinafter referred to as the "prior art 2").
However, the prior arts 1 and 2 described above have the following problems:
It is true that the cold-rolled steel sheets of the prior arts 1 and 2 are excellent in workability and have a high tensile strength of over 100 kgf/mm.sup.2. An ultra-high-strength cold-rolled steel sheet having a tensile strength of over 100 kgf/mm.sup.2 is usually formed through the bending. In the cold-rolled steel sheets of the prior arts 1 and 2, however, when the tensile strength of the steel sheet becomes higher over 100 kgf/mm.sup.2, a fracture phenomenon (hereinafter referred to as the "delayed fracture") is suddenly caused by hydrogen penetrating into the interior of the steel sheet under the effect of a corrosion reaction taking place along with the lapse of time at a portion formed by the above-mentioned bending of the cold-rolled steel sheet. Therefore, even with a high tensile strength, a cold-rolled steel sheet susceptible to the delayed fracture, has a fatal defect as a material for protective components of an automobile, for example.
Under such circumstances, there is a strong demand for the development of an ultra-high-strength cold-rolled steel sheet excellent in the property inhibiting the occurrence of delayed fracture (hereinafter referred to as "delayed fracture resistance") and having a high tensile strength of over 100 kgf/mm.sup.2 and a method for manufacturing same, but such an ultra-high-strength cold-rolled steel sheet and a method for manufacturing same have not as yet been proposed.
An object of the present invention is therefore to provide an ultra-high-strength cold-rolled steel sheet excellent in delayed fracture resistance and having a high tensile strength of over 100 kgf/mm.sup.2 and a method for manufacturing same.